Intervertebral disc prosthesis

ABSTRACT

An intervertebral-disc prosthesis for insertion into an intervertebral disc compartment has a first and a second constituent element ( 10,10 ′) each being provided on one side with a joint member ( 28,28 ′) and on the other side with an abutment face which abuts against adjacent vertebrae. The abutment faces have a convexly curved region that has at least substantially the shape of a ramp. Preferably the region is surrounded by an annular flat region having a larger roughness than the curved region. The curved region may have a vertex which contacts a vertex of a dome formed in the adjacent vertebra. This facilitates rotation of the constituent element within the intervertebral disc compartment.

CROSS-REFERENCE TO RELATED APPLICATIONS

This US national phase application is based on international applicationno. PCT/EP2006/006609 filed on Jul. 6, 2006 and claims priority benefitof U.S. provisional application Ser. No. 60/696,882 filed Jul. 6, 2005and Ser. No. 60/741,817 filed Dec. 2, 2005 and claims priority toEuropean patent application EP 06002765.3 filed Feb. 10, 2006. The fulldisclosure of these earlier applications is incorporated herein byreference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to intervertebral disc prostheses forinsertion into an intervertebral disc compartment which is formedbetween a first and a second vertebra. More particularly, the inventionrelates to intervertebral disc prostheses that restores the naturalflexibility of the spine.

2. Description of Related Art

Intervertebral disc prostheses generally comprise two constituentelements each having a supporting plate that rests with an abutment faceon an adjacent vertebra when the prosthesis is inserted into theintervertebral disc compartment. The other side of the respectivesupporting plate supports a joint member that allows a relative movementbetween the two supporting plates.

U.S. Pat. No. 6,936,071, which corresponds to WO 01/07893 A1, disclosesan intervertebral disc prosthesis in which the upper supporting platecomprises a cap-shaped recess. The lower supporting plate is providedwith a recessed compartment that receives an exchangeable slide-ininsert having a cap-shaped projection. The cap-shaped recess in theupper supporting plate and the cap-shaped projection of the insert forma ball-and-socket joint so that the two supporting plates can be tiltedrelative to one another in all directions in space.

Before the intervertebral disc prosthesis is inserted into a cervicalintervertebral disc compartment, the latter is usually machined in amaterial-abrading manner. During this procedure bone material is abradedwith a milling cutter in order to create appropriately large and planeopposite surfaces on the adjoining vertebrae for the abutment faces ofthe supporting plates. Fins formed on the abutment faces of thesupporting plates prevent slipping of the otherwise plane abutment faceson the adjacent vertebrae. The fins engage grooves which have previouslybeen recessed into the adjoining vertebrae by chiseling. The precisemilling of the grooves, however, is a relatively elaborate process. Forthat reason it is difficult to correctly position the prosthesis in theintervertebral disc compartment.

WO 2005/004756 A1 discloses an intervertebral disc prosthesis that doesnot comprise fins on the abutment faces. Instead, the abutment faces areslightly convexly curved in a manner that is adapted to the anatomicrequirements of the intervertebral disc prosthesis. The geometric dataof the healthy intervertebral disc compartment, in particular itsheight, are determined by extrapolation of data obtained from 3Dscanning measurements performed on the diseased spine section. Theconvex curvature of the abutment faces is said to result in aself-centering effect of the supporting plates within the intervertebraldisc compartment. The abutment faces are coated with hydroxyl apatit(HAK) ceramics or TCP materials. These coatings are usually porous andrough which ensures that the supporting plates do not slide within theintervertebral disc compartment.

With such abutment faces it is difficult to obtain both the desiredself-centering effect and a durable fixation of the prosthesis in theintervertebral disc compartment. If the abutment faces have a coatingwith a smooth surface, the supporting plates slide within theintervertebral disc compartment. If the abutment faces have a coatingwith a rough surface, the friction is too large to obtain aself-centering effect.

EP 0 754 018 B1 discloses a similar intervertebral disc prosthesis. Herethe center of motion of the prosthesis, i.e. the center of curvature ofthe aspherical ball-and-socket joint, is located in the posterior partof the prosthesis, but still between the supporting plates.

WO 03/090648 A1 discloses another prosthesis in which each supportingplate is configured to receive one insert. Each insert is provided witha cap-shaped recess. Both recesses commonly accommodate a ball enablingarticulating movement between the supporting plates. The supportingplates have cylindrically curved abutment faces and flat wings arrangedat the lateral sides of the supporting plates. Before the prosthesis canbe inserted, the adjacent vertebrae have to be prepared by formingelongate recesses in the bone material. The shape of the recessescorresponds to the cylindrical shape of the abutment faces of thesupporting plates. In order to prevent sliding movements of thesupporting plates relative to the vertebrae along the cylinder axes, thewings are provided with teeth that improve the grip of the wings on thebone material.

Also with this prosthesis its positioning within the intervertebral disccompartment is solely determined by the skill of the surgeon whoprepares the cylindrical recesses that accommodate the abutment faces ofthe supporting plates.

SUMMARY OF THE INVENTION

It is a first object of the present invention to provide anintervertebral disc prosthesis which is easy to implant into anintervertebral disc compartment and nevertheless remains fixedly in itsimplant position.

According to a first aspect of the invention, this object is achieved byan intervertebral disc prosthesis for insertion into an intervertebraldisc compartment which is formed between a first vertebra and a secondvertebra. The prosthesis comprises a first supporting plate which isprovided on one side with a first joint member and has on the other sidea first abutment face which abuts against the first vertebra when theintervertebral disc prosthesis is inserted into the intervertebral disccompartment. A second supporting plate is provided on one side with thesecond joint member and has on the other side a second abutment facewhich abuts the second vertebra when the intervertebral disc prosthesisis inserted into the intervertebral disc compartment. At least one ofthe first and second abutment faces has a convexly and asphericallycurved region. According to the invention the curved region iscompletely surrounded by an annular flat region which has a roughersurface than the curved region.

The provision of a relatively smooth surface of the convexly curvedregion makes it possible that the prosthesis adjusts itself with respectto the domes by sliding movements when the intervertebral discprosthesis is inserted into the intervertebral disc compartment. If thesmooth surface is not rotationally symmetrical, but has a long and ashort dimension in orthogonal directions, such a smooth surface evenmakes it possible that the intervertebral disc prosthesis may beinserted through a lateral access. A lateral access is often preferredbecause it does not require to push aside large and sensitive bloodvessels that block the ventral access channel.

During insertion the intervertebral disc prosthesis is inserted throughthe access channel preferably with its long dimension parallel to thechannel axis. After the intervertebral disc prosthesis has reached aposition between the adjacent vertebrae, the pressure exerted by thevertebrae on the polished surface causes the prosthesis to rotate byabout 45° to 90° (depending on the direction of the access channel) inthe intervertebral disc compartment until it finally reaches its desiredposition with its long dimension extending laterally. Alternatively,operating means connected to the prosthesis are used to rotate theprosthesis within the intervertebral disc compartment.

The curved region is formed such that it, after having been insertedinto the intervertebral disc compartment from a ventral or lateralaccess channel, further penetrates for some millimeters into the porousand relatively soft bone tissue (substantia spongiosa) between the tubeof harder bone (substantia compacta). The annular ends of this hard tubewill be referred to in the following as “apophyseal ring” of thevertebrae. The penetration is stopped when a flat and preferably roughregion surrounding the convexly curved region rests on the apophysealring of the adjacent vertebra. This ensures a very tight and reliableconnection of the intervertebral disc prosthesis to the adjacentvertebra.

In an advantageous embodiment the curved region of at least one abutmentface is formed with a vertex point or vertex area being configured suchthat the constituent element is allowed to rotate within the adjacentdome by at least 10°, preferably by at least 25°, when the vertex of thecurved region contacts a vertex of the dome. With abutment faces havingexactly the shape of the dome, it is difficult for the surgeon tomanually rotate the constituent elements with respect to the adjacentvertebra, because large curved faces are in immediate contact. If theabutment face is provided with such a vertex, however, only the verticesof the abutment faces and the domes contact each other before a moreintimate connection is achieved after the abutment face has been pressedinto the soft bone material within the dome.

Such a shape of the convexly curved region may require that the shape ofthe dome is biometrically determined prior to the insertion of theintervertebral disc prosthesis into the intervertebral disc compartment.This may be accomplished by computer processing high-resolution imagesof the vertebrae. The curved region is then machined or molded inaccordance with the obtained biometrical shape data of the dome.However, since the shape of the curved region is preferably similar, butnot identical to the geometry of the dome within the apophyseal ring,such an adaptation to the patient's specific geometry may in many casesnot necessary. Instead, supporting plates that are fabricated on thebasis of statistical data obtained for the affected vertebrae of manypatients may be used.

There are many approaches to achieve a smooth surface of the curvedregion. For example, it may be polished or provided with a coating whichhas the desired surface properties. Preferably the surface has aarithmetic roughness Ra of less than 10 μm, preferably less than 1 μmwhich ensures that the kinetic friction coefficient is less than 0.1with respect to the bone material.

Such properties may easily be achieved with the application of adiamond-like carbon coating (DLC) which is known in the art as such.Such coatings are biologically compatible, are very hard and have a lowfriction.

On the other hand, the flat region may have a kinetic frictioncoefficient of more than 1.0 with respect to the bone material of theapophyseal ring.

Of course, both of the supporting plates may also be formed in themanner described above.

According to another aspect of the invention, the above stated object isachieved by an intervertebral disc prosthesis for insertion into anintervertebral disc compartment which is formed between a first vertebraand second vertebra. The prosthesis comprises a first supporting platewhich is provided on one side with a first joint member and has on theother side a first abutment face which abuts against the first vertebrawhen the intervertebral disc prosthesis is inserted into theintervertebral disc compartment. It further comprises a secondsupporting plate which is provided on one side with a second jointmember and has on the other side a second abutment face which abuts thesecond vertebra when the intervertebral disc prosthesis is inserted intothe intervertebral disc compartment. At least one of the first andsecond abutment faces has a convexly curved region. According to theinvention the convexly curved region has (at least substantially) thecase of a ramp.

It has been discovered that such a shape is similar to the geometry ofthe dome that accommodates the curved region if the supporting platewith the curved region is implanted. Since the ramp is not rotationallysymmetrical, a self-centering torque is produced if there is acompressive force applied between the vertebra and the supporting plate.During the implant surgery this force is produced by ligaments thatextend along the spine. When the supporting plate has rotated to aposition where these compressive forces are symmetrical, the torquevanishes and the rotation ceases accordingly. If the compressive forceapplies still further, the ramp-shaped curved region will press into thebone material (substantia spongiosa) within the apophyseal ring, therebycausing a partial deformation of the bone material. This ensures a veryintimate contact between the bone material and the curved.

Even if the implant of an intervertebral disc prosthesis has beenperformed without any complications, it may nevertheless sometimes benecessary to change the joint members that determine the location of thecenter of motion. Usually the prosthesis will be implanted through aventral access. However, creating such a ventral access for a secondtime is often associated with increased risks due scars caused by theformer surgery.

It is therefore another object of the present invention to provide anintervertebral disc prosthesis that reduces such risks.

According to the invention, this object is achieved by an intervertebraldisc prosthesis for insertion into an intervertebral disc compartmentwhich is formed between a first and a second vertebra. The prosthesiscomprises a first supporting plate having a longitudinal side and atransfer side, a first slide-in compartment on the first supportingplate, a first joint-member which is separately connected to the firstsupporting plate and comprises a first slide-in plate which is capableof being inserted into the first slide-in compartment. The prosthesisfurther comprises a second supporting plate which supports a secondjoint member. According to the invention, the first slide-in plate andthe first slide-in compartment are configured such that the firstslide-in plate is capable of being inserted into the first slide-incompartment both from the longitudinal side and from the transfer sideof the first supporting plate.

This makes it possible to exchange the first joint element from alateral access channel. Any risks associated with providing a ventralaccess channel for a second time are thus avoided.

The inventor has discovered that many of the problems encountered withconventional intervertebral discs prosthesis are a result of a mismatchbetween the anatomically possible movements of the vertebrae on the onehand and the movements made possible by the prosthesis. If this mismatchis substantial, the muscles and ligaments supporting the spine arenaturally strained, which results in tenseness and finally in pain.

It is therefore a further object of the present invention to provide anintervertebral disc prosthesis which reduces this mismatch considerably.

According to the invention this object is achieved by an intervertebraldisc prosthesis for insertion into an intervertebral disc compartmentwhich is formed between an upper and a lower vertebra. The prosthesiscomprises a first joint member that is associated with the uppervertebra. A second joint member is associated with the lower vertebraand is configured such that it is capable of swiveling relative to thefirst joint member around a swivel point or swivel axis. According tothe invention the swivel point or swivel axis is positioned above thesecond joint member and preferably above the first joint member. In manycases it may even be advantageous to position this swivel point orswivel axis above a supporting plate that supports on one side the firstjoint member and has on the other side a first abutment face which abutsagainst the first vertebra when the intervertebral disc prosthesis isinserted into the intervertebral disc compartment.

In conventional prostheses, the swivel point is positioned within orbelow the lower joint member. However, it has been discovered that thecenter of motion is not below, but (significantly) above the lower jointmember. For example, in the case of lumbar vertebrae the center ofmotion is in the apex of the dome that is surrounded by the apophysealring of the upper vertebra. Only if the swivel point or swivel axis ofthe prosthesis substantially coincides with the anatomical center ofmotion, the elements of the prosthesis will swivel to an extent as it isthe case if a natural and healthy intervertebral disc is in theintervertebral disc compartment. As a result, unnatural strain ofligaments and muscles are avoided, and a stiffening of the prosthesiswill not occur.

In order to be able to position the swivel point or swivel axis as closeas possible to the natural center of motion of the adjacent vertebrae,the position and curvatures of the joint elements have to be carefullyselected. For that reason the prosthesis may be assembled like aconstruction kit from a plurality of components of the same kind, buthaving different dimensions or curvatures. This makes it possible thatthe surgeon assembles the prosthesis during the implant surgery when heis able to determine how much bone material has to be removed from theadjacent vertebra. As a matter of course, the assembly of the prosthesisfrom a variety of different components is also advantageous if no oronly a small amount of bone material has to be removed, as is it oftenthe case with lumbar vertebrae, or if it is clear in advance how muchbone material has to be removed. In these cases the components aredetermined on the basis of preoperative 3D scans such that the swivelpoint or swivel axis will be as close as possible to the natural centerof motion of the adjacent vertebrae.

Such a construction kit may comprise first and second joint membershaving a different shape (curvatures and/or spacings from basesurfaces), and/or different supporting plates and/or a set of differentspaces having the shapes of discs or wedges that can be inserted betweenthe supporting plates and the adjacent vertebrae.

A further problem with the known intervertebral disc prosthesesdescribed further above consists in the fact that the mobility of thesupporting plates in the event of a lateral tilting of the vertebraeabout a tilt axis parallel to the transverse sides of the supportingplates is limited only by the fact that the edges of the supportingplates strike one another in the case of a relatively large tilt angle.Occasionally it is also desirable to be able to limit the tilt angle ofthe supporting plates towards the front and/or towards the backselectively.

For this reason it is a further object of the invention to provide anintervertebral disc prosthesis in which tilt angles can be limited in astraightforward manner.

According to the invention this object is achieved by an intervertebraldisc prosthesis for insertion into an intervertebral disc compartmentwhich is formed between a first and a second vertebra. The prosthesiscomprises a first supporting plate supporting a first joint member and asecond supporting plate supporting a second joint member. The secondjoint member cooperates with the first joint member so as to allow atilting or swiveling movement between the first and second supportingplates. A projection is arranged on the first supporting plate whichserves as a stop that restricts the relative tilting or swivelingmovement between the first and second supporting plates. According tothe invention the projection is arranged on an exchangeable guide-platewhich is capable of being inserted into the guide-plate slide-incompartment provided on the first supporting plate.

By providing one or more projections on guide-plates it is possible toeasily exchange the projections by merely removing a guide-plate andinserting another guide-plate having a different projection. Since theinsertion into a slide-in compartment requires only a translationalmovement, such an exchange may be carried out through a very smallaccess channel if the orientation of the slide-in compartment isappropriately selected with respect to the possible orientations ofaccess channels. Often a lateral access is preferable once anintervertebral disc prosthesis has been implanted. For that reason theslide-in compartments should be oriented such that the guide-in platesupporting the projection can be drawn out the slide-in compartment witha lateral movement.

A limitation of the lateral tilt angles is achieved if the firstprojection is arranged between the first joint member and a transverseside of the first supporting plate. If the tilt angle of the supportingplates towards the front and/or towards the back is to be limited, theprojection should be arranged between the first joint member and alongitudinal side of the first supporting plate.

A further problem with the known intervertebral disc prostheses is thata relatively large ventral access canal has to be prepared for theinsertion of the supporting plates.

For this reason, a further object of the present invention is to providean intervertebral disc prosthesis that can be inserted into theintervertebral disc compartment through a smaller ventral access canal.

This object is achieved by means of an intervertebral disc prosthesiscomprising a first supporting plate which supports a first joint memberand with a second supporting plate which supports a second joint member.According to the invention, the first supporting plate comprisesconnecting elements for separably connecting the first supporting plateto an operating means with which the first supporting plate can berotated about an axis that is substantially perpendicular to the firstsupporting plate.

In this way, the first supporting plate, with its short transverse sidepointing forward, can be introduced through a narrower ventral accesscanal having a diameter that is given not by the length of the longsides, but by the length of the short narrow transverse sides of thesupporting plates. The supporting plate is firstly rotated into thedefinitive position in, or in the vicinity of, the intervertebral disccompartment with the aid of the operating means. The smaller thediameter of the access canal, the lower is the risk of damage to bloodvessels that have to be displaced when the ventral access canal is beingcreated.

BRIEF DESCRIPTION OF THE DRAWINGS

Various features and advantages of the present invention may be morereadily understood with reference to the following detailed descriptiontaken in conjunction with the accompanying drawing in which:

FIG. 1 is a top view of an upper constituent element of anintervertebral disc prosthesis according to the invention;

FIG. 2 is a section along line II-II of the upper constituent elementshown in FIG. 1;

FIG. 3 is a bottom view of the upper constituent element shown in FIG.1;

FIG. 4 is a section along line IV-IV of the upper constituent elementshown in FIG. 1;

FIG. 5 is a section along line V-V of the upper constituent elementshown in FIG. 1;

FIGS. 6 a to 6 c show the underside of the upper supporting plate inseveral states during the assembly of the upper constituent element;

FIG. 7 is a top view of a lower constituent element of an intervertebraldisc prosthesis according to the invention;

FIG. 8 is a section along line VIII-VIII of the lower constituentelement shown in FIG. 7;

FIG. 9 is a bottom view of the lower constituent element shown in FIG.7;

FIG. 10 is a side view of the intervertebral disc prosthesis in theinserted state;

FIG. 11 is a side view of the intervertebral disc prosthesiscorresponding to FIG. 10 but with constituent elements tilted relativeto one another;

FIG. 12 is a side view of an intervertebral disc prosthesis,corresponding to FIG. 10, which has been fixed so as to counter lateraltilting movements;

FIG. 13 is a representation, corresponding to FIG. 2, of a variantwithout projections serving as stops;

FIG. 14 is a schematic simplified sectional view of an upper and a lowerlumbar vertebra;

FIG. 15 shows the joint elements of the two constituent elements in afirst configuration;

FIG. 16 shows the joint elements of the two constituent elements in asecond configuration;

FIG. 17 shows the joint elements of the two constituent elements in athird configuration;

FIG. 18 shows the joint elements of the two constituent elements in afourth configuration;

FIG. 19 shows the joint elements of the two constituent elements in afifth configuration;

FIG. 20 shows the joint elements of the two constituent elements in asixths configuration;

FIG. 21 is a side view of an intervertebral disc prosthesis according toanother embodiment inserted between two lumbar vertebrae;

FIGS. 22 a to 22 d show the underside of the upper supporting plate inseveral states during the exchange of a joint element;

FIGS. 23 a to 23 c are top views of the upper supporting plate which isrotated with the aid of operating rods during its introduction into aventral access canal;

FIG. 24 is a perspective schematic representation of a connectingelement for the operating rods;

FIG. 25 is a side view of an intervertebral disc prosthesis according toa further embodiment inserted between two lumbar vertebrae in a firststate;

FIG. 26 shows the prosthesis of FIG. 25, but in a second state afterswiveling the lower constituent element with respect to the upperconstituent element;

FIG. 27 is a side view of an intervertebral disc prosthesis according toa still further embodiment inserted between two lumbar vertebrae in afirst state; and

FIG. 28 shows the prosthesis of FIG. 27, but in a second state afterswiveling the lower constituent element with respect to the upperconstituent element;

FIG. 29 is a top view of an upper constituent element of anintervertebral disc prosthesis according to another embodiment of theinvention;

FIG. 30 is a section along line XXX-XXX of the upper constituent elementshown in FIG. 29;

FIG. 31 is an enlarged section through a cap insert element of the upperconstituent element shown in FIGS. 29 and 30;

FIG. 32 is a section through the cap insert shown in FIG. 31 along lineXXXII-XXXII;

FIG. 33 is an enlarged section through a cap insert element according toanother embodiment;

FIG. 34 is a section through the cap insert shown in FIG. 33 along lineXXXIV-XXXIV;

FIGS. 35 a to 35 c are cross sections through the cap insert shown inFIG. 33 and an adjacent vertebra in various constellations during theimplant procedure;

FIGS. 36 a to 36 c are cross sections through the cap insert shown inFIG. 34 and an adjacent vertebra in various constellations during theimplant procedure;

FIG. 37 is an enlarged section through a cap insert element according tostill another embodiment.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

FIGS. 1 to 5 show an upper constituent element of an intervertebral discprosthesis in a top view, in a section along line II-II, in a bottomview, in a section along line IV-IV, and in a section along line V-V,respectively.

The upper constituent element denoted in its entirety by 10 comprises anupper supporting plate 12 having a periphery that is approximatelykidney-shaped. Into a central recess 14 (see FIG. 2) on the upper sideof the upper supporting plate 12 a spherical-cap insert 16 is insertedand is held there by a latching ring 18 which is formed on thespherical-cap insert 16. In the exemplary embodiment shown theoutward-pointing external surface, denoted by 20, of the spherical-capinsert 16 is curved in spherically convex manner. However, inserts withaspherical convex caps may be used as well, as is described furtherbelow with reference to FIGS. 24 to 27. The external surface 20 of thespherical-cap insert 16 supports a coating 22 that has a high degree ofroughness.

In the exemplary embodiment shown the spherical-cap insert 16 consistsof a relatively soft and elastic material, for example an elastomer,whereas the surrounding upper supporting plate 12 is manufactured from aharder material, for example a metal such as titanium. After theinsertion of the upper constituent element into an intervertebral disccompartment, the spherical-cap insert 16 sits close, with its roughcoating 22, on the relatively soft material which is located within theapophyseal ring of the adjacent vertebra. The surrounding harder regiondenoted by 23 on the outward-pointing side of the upper supporting plate12 is supported on the apophyseal ring of the adjacent vertebra.

On the underside of the upper supporting plate 12 situated opposite thespherical-cap insert 16 there is a joint element 24 which comprises aslide-in plate 26 with a convex spherical-cap joint 28 integrally moldedthereon. Parallel to the transverse sides of the upper supporting plate12 there extend guide ribs 30, 32 which are molded on the slide-in plate26. Due to the convex curvature of the spherical-cap joint 28, thecenter of motion is positioned above the joint. This has substantialadvantages at least for prostheses inserted in lumbar intervertebralcompartments, as will be explained in more detail below.

In the direction towards the transverse sides of the upper supportingplate 12 the joint element 24 adjoins a first guide plate 34 and asecond guide plate 36. The two guide plates 34, 36, which in principleare of similar construction, bear a first and a second projection 38,40, respectively, which are of parallelipipedal design and which serveas stops for the purpose of limiting the movement of the joint. In thedirection towards the joint element 24 the guide plates 34, 36 areprovided with guide grooves 42, 44 which are engaged by the guide ribs30, 32 of the slide-in plate 26 of the joint element 24.

As can be discerned in the sectional representation along line V-V shownin FIG. 5, in the direction towards the longitudinal sides of the uppersupporting plate 12 the two guide plates 34, 36 are likewise providedwith guide grooves 46, 48 which are of a design that is complementary toguide ribs 50, 52 which are formed on the upper supporting plate 12.

In FIG. 5 it can further be discerned that the second guide plate 36 isreceived in a second guide-plate slide-in compartment 54 which is openonly in the direction towards the middle of the upper supporting plate12. The second guide plate 36 can consequently be inserted from the sideinto the second guide-plate slide-in compartment 54, with the guidegrooves 46, 48 of the second guide plate 36 engaging the guide ribs 50,52 of the upper supporting plate 12. The front side of the second guideplate 36, on which the guide groove 44 is formed, finally strikes a stop55 which is formed on the upper supporting plate 12 (see FIG. 3). Inthis way, the position of the second guide plate 36 in the secondguide-plate slide-in compartment 54 is defined by abutment in alldirections other than the slide-in direction.

In order to prevent an unintentional slipping of the second guide plate36 out of the second guide-plate slide-in compartment 54, the secondguide plate 36 is preferably manufactured from a material that haselastic properties. Polyvinyl chloride, for example, may be envisagedfor this purpose. The frictional resistance between the second guideplate 36 and the upper supporting plate 12 should be such that thesecond guide plate 36 can be inserted into the second guide-plateslide-in compartment 54 by hand or with the aid of a tool but cannot beredetached from the second guide-plate slide-in compartment 54 withoutthe aid of a tool after the insertion of the intervertebral discprosthesis into the intervertebral disc compartment.

As FIGS. 1 to 3 show, the upper constituent element 10 is formedsymmetrically relative to a plane of symmetry passing through the middleof the spherical-cap insert 16 and of the spherical-cap joint 28. Thefirst guide plate 34 is consequently constructed in the same way as thesecond guide plate 36, and thus the above remarks apply correspondinglyto the second guide plate 36.

The side faces of the two guide plates 34, 36 pointing towards themiddle define a slide-in compartment 56 for the joint element 24 alongthe transverse sides of the upper supporting plate 12. Along thelongitudinal sides of the upper supporting plate 12, the slide-incompartment 56 is delimited, on one side, by a step 58 which is formedon the upper supporting plate 12 (cf. FIG. 3) and, on the longitudinalside situated opposite, by a third guide plate 60. The third guide plate60 is inserted into a third guide-plate slide-in compartment 62 which isformed on the underside of the upper supporting plate 12. The side ofthe front guide plate 60 pointing towards the middle of the supportingplate 12 is, as can be discerned in FIG. 4, likewise provided with aguide groove 63 which is engaged by a guide rib 65, of complementaryshape, pertaining to the slide-in plate 26 of the joint element 24.

In this way, the slide-in plate 26 of the joint element 24 bears onthree sides against the guide plates 34, 36 and 60 and on the fourthside against the step 58 and is consequently fixed on the underside ofthe upper supporting plate 12 in all directions.

In FIGS. 6 a to 6 c the assembly of the upper constituent element 10 iselucidated in several stages in a simplified schematic representation.

In FIG. 6 a the underside of the upper supporting plate 12 is shownwithout the guide plates 34, 36, 60 and the joint element 24. In thisrepresentation, lateral guide-plate slide-in compartments 64, 66 for thefirst and second guide plates 34, 36, respectively, and also the thirdguide-plate slide-in compartment 62 for the third guide plate 60 can bediscerned. As indicated by arrows 68, 70, the first and second guideplates 34, 36 are now inserted from the side into the guide-plateslide-in compartments 64 and 66, respectively, until they strike thestops 55, 59.

This state is shown in FIG. 6 b. The lateral guide plates 34, 36 nowform, together with the step 58, the slide-in compartment 56 for theslide-in plate 26 of the joint element 24. The slide-in plate 26 is nowinserted into the slide-in compartment 56 from the front, as indicatedby an arrow 72. In order to secure the joint element 24 againstunintentional slipping out of the slide-in compartment 56, the thirdguide plate 60 is now also inserted into the slide-in compartment 56from the front, as indicated in FIG. 6 c by an arrow 74.

The modular structure, described above, of the upper constituent element10 permits the joint element 24 and the first and second guide plates34, 36 with the projections 38 and 40, respectively, serving as stops tobe assembled practically arbitrarily in the manner of a constructionkit. Furthermore, an exchange of the guide plates 34, 36 and of thejoint element 24 is possible even after the intervertebral discprosthesis has been surgically inserted into the intervertebral disccompartment. This will be elucidated in more detail further below withreference to FIGS. 12 to 19.

FIGS. 7 to 9 show, in representations similar to FIGS. 1 to 3, a topview, a section along line VIII-VIII, and a bottom view of a lowerconstituent element 10′ which together with the upper constituentelement forms the intervertebral disc prosthesis. The lower constituentelement 10′ differs from the upper constituent element 10 merely in thatthe convex spherical-cap joint 28 has been replaced by a concavespherical-cap joint 28′. The curvatures of the convex spherical-capjoint 28 and of the concave spherical-cap joint 28′ are numericallyequal, so the two spherical-cap joint 28, 28′ together form aball-and-socket joint.

Since the two constituent elements 10, 10′ are otherwise of identicalconstruction, reference may to this extent be made to the description ofthe upper constituent element 10. In order to make parts of the upperconstituent element 10 and of the lower constituent element 10′corresponding to one another distinguishable, the parts of the lowerconstituent element 10′ are labeled with dashed reference numerals.

FIG. 10 shows an intervertebral disc prosthesis constructed from the twoconstituent elements 10, 10′ after its positioning in an intervertebraldisc compartment. In FIG. 10 it can be readily discerned that therelatively soft and elastic spherical-cap insert 16 with the roughcoating 22 bears against the likewise soft material that is locatedwithin the apophyseal ring of an upper vertebra V1. In this way, a veryintimate connection is achieved between the spherical-cap insert 16 andthe soft material of the vertebra V1. The roughness of the coating 22has the effect that the upper constituent element is unable to slipwithin the intervertebral disc compartment.

The harder region of the upper supporting plate 12 surrounding thespherical-cap insert 16 is supported on the apophyseal ring 71 of thevertebra V1. Via the harder region, the large longitudinal forces aretransmitted between the vertebra V1 and the upper constituent element10. Corresponding remarks apply in respect of the lower constituentelement 10′, which rests on the lower vertebra V2.

In the inserted state the convex spherical-cap joint 28 of the upperconstituent element 10 rests on the concave spherical-cap joint 28′ ofthe lower constituent element 10′, so that the two constituent elements10, 10′ can be swiveled in all directions relative to one another abouta swivel point 80 whose significance will be described in more detailbelow. Since a swiveling of the vertebrae V1, V2 in the lateraldirection is generally intended to be possible only to a limited extent,the projections 38, 40 on the guide plates 34, 36 of the upperconstituent element 10 and also the corresponding projections 38′, 40′on the guide plates 34′, 36′ of the lower constituent element 10′ areconstructed in such a way that the two constituent elements 10, 10′ canonly be swiveled by a few degrees about an axis perpendicular to theplane of the paper. In FIG. 11 it is shown how the projections 38, 38′limit a swiveling movement to a maximal swivel angle of about 4°. Itshould be noted that it may suffice to provide only one of theconstituent elements 10, 10′ with projections.

As can be seen in FIGS. 3 and 9, the projections 38, 40 and 38′, 40′have a rectangular base area, with the long side extending in adorsal-ventral direction. This shape ensures also a limitation ofswiveling movements forward and backward. The length of the long sidedetermines the maximum swivel angle in this direction which may be inthe range between 6° and 12°. With projections 38, 40 and 38′, 40′having a substantially quadratic base area, no limitation of such swivelmovements is achieved, which may be desirable under certaincircumstances.

If the maximal swivel angle for one or both swivel axes is to bechanged, it is sufficient to exchange either the upper guide plates 34,36, the lower guide plates 34′, 36′, or all four guide plates 34, 36,34′, 36′ for guide plates that are provided with different projections.In this case the projections 38, 40 and 38′, 40′ either may have adifferent height, a different base area or may be arranged closer to, orfurther away from, the joint elements 24, 24′, for example. It should benoted that such changes of the swivel angles are possible even after thedisc prosthesis has been inserted into the intervertebral compartment.This is because the guide plates 34, 36, 34′, 36′ can be replacedthrough lateral accesses without the need to remove the entireprosthesis.

FIG. 12 shows a representation corresponding to FIGS. 10 and 11, inwhich guide plates 134, 136, 134′, 136′ have been inserted into theupper and lower supporting plates 12, 12′, respectively, the projectionsof which, 138, 140 and 138′, 140′, respectively, totally fix theintervertebral disc prosthesis. For this purpose the projections 138,140, 138′, 140′ may have relatively large-area and flat bearing faces inorder to achieve a good distribution of forces. If the intervertebraldisc prosthesis is to be fixed merely in the lateral direction, butswiveling movements towards the front and towards the rear are tocontinue to be possible, the projections 138, 140, 138′, 140′ may takethe form of transversely situated semicylinders. The semicylinders arethen able to roll onto one another, so that a swiveling of the twoconstituent elements 10, 10′ is possible about a swivel axis which isindicated in FIG. 12 by 73.

In order to limit the maximal swivel angle for swiveling movements inthe forward direction, one or both third guide plates 60, 60′ may alsobe provided with projections which serve as stops. However, theprovision of the third guide plates 60, 60′ with projections may not bepreferred if the projections shall be replaceable. This is because theguide plates 60, 60′ can only be exchanged through a ventral accesscanal, and the preparation of such a canal for a second time is often adifficult and risky process.

Of course, the stops limiting the mobility may also be dispensed withaltogether. In this case, use is made of guide plates 234, 236 withoutprojections. FIG. 13 shows such an embodiment in a sectionalrepresentation similar to FIG. 2.

It is to be understood that the intervertebral disc prosthesis describedabove can also be inserted into an intervertebral disc compartment theother way round, so that the upper constituent element 10 adjoins thelower vertebra V2 and the lower constituent element 10′ adjoins theupper vertebra V1.

Since the joint elements 24, 24′ of the two constituent elements 10, 10′are exchangeable in a straightforward manner, the intervertebral discprosthesis can be adapted within very wide limits to the respectiveshape of the intervertebral disc compartment and to the possiblemovements of the vertebrae V1, V2 adjoining it.

The shape of the intervertebral disc compartment usually variesdepending on the longitudinal position of the adjacent vertebrae in thehuman spine. Often, however, the size of the compartment has to besubstantially enlarged by abrasive methods, for example for removingpathological deformations in the vicinity of an apophyseal ring of oneor both adjacent vertebra.

The possible movements of the vertebrae are anatomically predeterminedby the facet joints, the joint capsules, the annulus fibrosus and theligaments that extend along the spine. The inventor has discovered thatmany of the problems encountered with conventional intervertebral discprostheses are a result of a mismatch between the anatomically possiblemovements of the vertebrae on the one hand and the movements madepossible by the prosthesis. If this mismatch is substantial, the musclesand ligaments supporting the spine are unnaturally strained, whichresults in tenseness and finally in pain. Further, it has been found outthat the movements of a pair of adjacent vertebrae may be described, atleast to a very good approximation, as a swiveling movement in which thelower vertebra is swiveled around a swivel point that is associated withthe upper vertebra.

For the purpose of describing the possible movements, use will be madein the following of the term ‘center of motion’, which denotes theswivel point of this movement. For cervical vertebrae, the center ofmotion of the vertebrae is a few millimeters away from the uppervertebral face which delimits the intervertebral disc compartment.Recent researches carried out by the inventor have shown, however, thatthis generally does not apply for the lumbar vertebrae. With these, thecenter of motion is positioned almost exactly in the apex of the lowerdome of the upper lumbar vertebra.

FIG. 14 shows in a schematic simplified sectional view an upper and alower lumbar vertebra LV1 and LV2, respectively, having domes that areapproximated in the sectional view by corners formed by straight lines.The possible movements of the upper lumbar vertebra LV1 is indicated inFIG. 14 by different broken lines. In this representation the center ofmotion, which is denoted by reference numeral 8, is situated in thecorner formed by the two straight lines of the lower corner of the upperlumbar vertebra LV1. This corner corresponds to the apex of the curveddome of a real upper lumbar vertebra. From the foregoing it becomesclear that an intervertebral disc prosthesis for a lumbar intervertebraldisc compartment should be configured such that its center of motion ispositioned as close as possible to the apex of the lower dome of theupper vertebra. With the prosthesis described above this may easily beachieved by inserting joint elements 24, 24′ having the required shape.

How it is possible to attain almost any arbitrary center of motion withthe prosthesis described above will now be explained in more detail withreference to FIGS. 15 to 18.

In FIG. 15 the two joint elements 24, 24′ of the two constituentelements 10 and 10′, respectively, are represented in isolated manner.The center of motion, which is denoted by 80, is the center of a dashedcircle 82 having a segment that coincides with the spherical surface 25of the spherical-cap joint 28 of the joint element 24.

Particularly in the case of cervical vertebrae it may be necessary toremove pathological deformations in the vicinity of an apophyseal ring.The removal of such deformations implies that the surgeon has to removea larger part of an vertebra adjacent the intervertebral disccompartment. Such a removal increases the height of the intervertebraldisc compartment. For example, if a larger portion of the lower part ofthe upper vertebra V1 has to be removed, this may be compensated with byreplacing the joint element 24 of the upper constituent element 10 by ajoint element 124 in which the spherical-cap face 125 is further awayfrom the slide-in plate 126 of the joint element 124.

This design is shown in FIG. 16 in a representation similar to FIG. 15.It can be discerned therein that the two slide-in plates 126, 126′ ofthe two joint elements 124, 124′ are now situated further apart, inorder to take account of the greater height of the intervertebral disccompartment. However, the joint element 124′ of the lower constituentelement 10′ has remained unchanged in comparison with the design shownin FIG. 15.

From FIG. 16 it becomes clear that, although the spherical-cap face 125is now further away from the slide-in plate 126, the position of thecenter of motion 180 has remained fixed with respect to the uppervertebra V1 at its optimal position which is anatomically predetermined.

Usually it is not possible for the surgeon to exactly estimate ahead ofthe implant surgery to which extent bone material of the adjacentvertebrae has to be removed. This usually becomes clear only during thesurgery. The surgeon may then measure the height of the resultingintervertebral disc compartment, for example by using an adjustabletemplate, and then determine at which position the supporting platesshould abut the vertebrae. Additional spacers arranged between thevertebrae and the supporting plates may be provided. If the position ofthe supporting plates is determined, the surgeon selects a pair ofslide-in plates having a geometry that ensures that the center of motionof the prosthesis coincides with the position of the anatomical centerof motion which has been determined beforehand on the basis of 3Dbiometrical measurements.

FIG. 17 shows a situation in which, other as shown in FIG. 16, not alarger part of the upper vertebra V1 but a larger part of the lowervertebra V2 had to be removed. One way to compensate for this is to usean upper joint element 224 having a larger distance between the slide-inplate 226 and the spherical face 225. In order to keep the position ofthe center of motion 280 at its optimal anatomically determinedposition, this requires also to increase the radius of curvature of thespherical face 225. The radius r is then a given function r=r(a) of thedistance a between the slide-in plate 226 and the spherical face 225. Inthis case both joint members 224, 224′ are different as compared to thesituation shown in FIG. 15.

FIG. 18 shows a combination of joint members 324, 324′ that might beused instead of the joint members 224, 224′ shown in FIG. 17. Here theradius of curvature of the spherical face 325 has not been increased.Instead, only the distance between the slide-in plate 326′ and thespherical face 325′ of the lower joint member 324′ has been increased,similar to the situation shown in FIG. 16 for the upper joint member124. Again, the center of motion 380 remains at the anatomicallypredetermined location in spite of the shifted positions of the abutmentfaces of the adjacent vertebrae V1, V2.

As an alternative to the exchange of joint members, the same effect canalso be achieved by an exchange of the supporting plates havingdifferent thicknesses. This is due to the fact that what ultimatelymatters for the position of the center of motion is not the spacing ofthe spherical faces from the slide-in plates, but rather their spacingfrom the abutment faces of the adjacent vertebrae V1, V2. Therefore itmight not even be necessary to exchange parts of the prosthesis as suchif additional spacers of varying thickness are inserted between theabutment sides of the supporting plates and the adjacent vertebra. Thespacers may have the shape of disks or wedges. A drawback of suchspacers is that they have to be kept in place, and apart from that theintervertebral disc compartment is often too small to insert additionalspacers.

By variation of the radii of curvature of the spherical-cap faces and/orof the spacings of the spherical-cap faces from the supporting plates itis therefore possible to adapt the intervertebral disc prosthesis tovirtually any arbitrary geometry of intervertebral disc compartmentswhilst retaining the anatomically predetermined center of motion. If thesurgeon is furnished with a kind of construction kit that comprises twosupporting plates and a set of differently shaped joint elements, thenby choice of the surgeon can perform the adaptation to the anatomicallypredetermined center of motion by making an appropriate choice of thejoint elements.

Alternatively, the inserting joint elements may be specifically machinedfor a particular patient. This makes it possible to retain the center ofmotion even in those cases in which the vertebrae are very stronglypathological deformed, for example due to an accident. In this case theupper and lower constituent elements may each be machined as one-pieceunit from a single block of a suitable material, e.g. titanium.

FIG. 19 shows an embodiment in which the center of motion 482 has beenshifted in a longitudinal direction. This is accomplished by shiftingthe spherical faces 425, 425′ of the slide-in plates 426, 426′ relativeto the base areas of these plates.

The same effect may be achieved by giving the spherical faces 525, 525′a shape that is not rotationally symmetric with respect to a centralaxis of the slide-in plates 526, 526′. Such an embodiment is shown inFIG. 20.

FIG. 21 shows, in a representation similar to FIG. 10, an intervertebraldisc prosthesis constructed from two constituent elements 210, 210′ inits position between two lumbar vertebrae LV1, LV2. The constituentelements 210, 210′ differ from the constituent elements 10, 10′ shown inFIGS. 1 to 11 only in that the spherical face 625 of the spherical-capjoint 628 and the spherical face 625′ of the recess 625′ have a largerradius of curvature. As a result, the center of motion 680 is positionedexactly in its anatomically optimum position, namely in the apex 690 ofthe dome 692 formed within the apophyseal ring 694 of the upper lumbarvertebra LV1.

Returning again to the first embodiment shown in FIGS. 1 to 11, itshould be noted that the first and second guide plates 34, 36 on thetransverse sides of the upper constituent element 10 make it possible totransfer the joint element 24 of the upper supporting plate 12 into itscentral position in the slide-in compartment 56 not only from the frontvia the third guide-plate slide-in compartment 62, but also from theside via one of the lateral guide-plate slide-in compartments 64, 66.Corresponding remarks also apply, of course, to the joint element 24′ ofthe lower constituent element 10′.

In particular, it is possible to exchange the joint element 24 from theside even when the intervertebral disc prosthesis has already beeninserted into the intervertebral disc compartment. The possibility ofbeing able to exchange the joint element 24 from the side in this caseis advantageous for the reason that a ventral access requires an accesscanal. However, creating such a canal for a second time is associatedwith increased risks. For instance, blood vessels that were displacedoutwards through the ventral access canal in the course of the firstoperation may scar. As a result of the scarring, the blood vessels losesome of their elasticity, so that serious complications may occur ifsaid blood vessels are displaced again in the course of a secondoperation. The creation of a lateral access canal is then the onlypossibility in order to be able to access an intervertebral discprosthesis that has already been inserted.

Such an access may be necessary, for example, if it turns out that theintervertebral disc prosthesis was not optimally adapted to the centerof motion and to the geometry of the intervertebral disc compartment.Furthermore, symptoms due to wear and tear of the joint elements mayoccur in rare cases, which impair the mobility of the intervertebraldisc prosthesis.

In the following it will be elucidated, on the basis of FIGS. 22 a to 22d, how in such a case the joint element 24 of the upper constituentelement 10 can be exchanged via a lateral access. Corresponding remarksalso apply, of course, to the joint element 24′ of the lower constituentelement 10′.

FIG. 22 a shows, in a representation based on FIG. 6 a, the underside ofthe upper supporting plate 12 pointing towards the lower constituentelement 10′. Via a lateral access canal leading to the intervertebraldisc compartment, firstly the second guide plate 36 is drawn out of thesecond guide-plate slide-in compartment 66, as indicated in FIG. 22 a byan arrow 84. For this purpose, a wire loop, for example, can be placedaround the projection 40 on the guide plate 36, with which the secondguide plate 36 can be drawn out of the second guide-plate slide-incompartment 66. The second guide-plate slide-in compartment 66 which isnow exposed reveals the lateral access to the joint element 24. Thejoint element 24 is now likewise drawn out via the second guide-plateslide-in compartment 66, as indicated in FIG. 22 b by an arrow 85. Forthis purpose, the slide-in plate 26 of the joint element 24 may beprovided with a bore 86. The surgeon is able to introduce a hook-shapedinstrument into the bore 86, with which he/she draws the joint element24 out laterally.

Another joint element 424 is now introduced again into the slide-incompartment 56, as indicated in FIG. 22 c by an arrow 88. Finally, thesecond guide plate 36 is reintroduced into the second guide-plateslide-in compartment 66, as indicated in FIG. 22 d by an arrow 90.

Of course, it is also possible, in the manner described above, toexchange not (only) the joint element 24 but (also) one or both guideplates 34, 36 via a lateral access, for example in order to fix theintervertebral disc prosthesis totally in the lateral direction, asshown in FIG. 12.

In order to keep the diameter of a ventral access canal small when theintervertebral disc prosthesis is being inserted, the supporting plates12, 12′ may firstly be introduced into the access canal with theirtransverse sides to the front, and may only be rotated into theirdefinitive position within the intervertebral disc compartment or in theimmediate vicinity thereof.

This will be shown in the following with reference to FIGS. 23 a, 23 band 23 c, which in exemplary manner show the upper supporting plate 12in top view. On the underside of the supporting plate 12 there arearranged connecting elements 94, 96, via which the upper supportingplate 12 can be connected separably and in articulated manner tooperating rods 98, 100. The connecting elements 94, 96 may be realizedas simple (pocket) bore into which the operating rods 98, 100 can beinserted. FIG. 24 shows an operating rod 98 which bears on one end ashort peg 97 bent at an angle of about 90°. The diameter of the peg 97is adapted to achieve a loose fit with a bore 99 provided in the firstsupporting plate 12. In this way, an easily separable and neverthelessarticulated connection is obtained between the upper supporting plate 12and the operating rods 98, 100.

The connecting elements 94, 96 are arranged at diagonally oppositecorners of the upper supporting plate 12. If the upper supporting plate12 is now connected to the operating rods 98, 100 via the connectingelements 94, 96, then as a result of moving the operating rods 98, 100the upper supporting plate 12 can be rotated about an axis that extendsperpendicular to a plane which is predetermined by the upper supportingplate 12.

If the operating elements 98, 100 are moved, for example in thedirection indicated by arrows 104, 106, the upper supporting plate 12rotates in the direction indicated by an arrow 108. This rotation iscontinued until such time as the upper supporting plate 12 attains theorientation shown in FIG. 23 b. In this orientation the upper supportingplate 12 requires a ventral access canal, the diameter of which has toamount merely to d′, where d′ is the maximal width of the uppersupporting plate 12. Indicated by d in FIG. 23 b is the diameter of aventral access canal such as would have to be created if the uppersupporting plate 12 were to be introduced into the access canal not withits transverse side to the front but with its longitudinal side to thefront.

As soon as the upper supporting plate 12 is located inside theintervertebral disc compartment, it is rotated back again into itsposition shown in FIG. 23 a. For this purpose, the operating rods 104,106 are moved in the direction indicated by arrows 110, 112, as a resultof which the upper supporting plate 12 rotates in the directionindicated by an arrow 114 into the final position shown in FIG. 23 c.The operating elements 98, 100 can now be separated from the connectingelements 94, 96 and drawn out of the ventral access canal.

FIG. 25 shows an intervertebral disc prosthesis according to anotherembodiment in a side view similar to FIG. 21. The prosthesis is insertedin an intervertebral disc compartment between two lumbar vertebrae LV1,LV2. In contrast to the embodiment shown in FIG. 21, the prosthesis hasno ball-and-socket joint between two constituent elements 610, 610′, buta needle joint comprising a recess 728 and a cone 728′ having a roundedtip. The cone 728′ thus has a shape similar to a sugar loaf.

The recess 728 is formed in a cap insert 716 of the upper constituentelement 710. An upper supporting plate 712 is provided with a centralopening 713, through which the tip of the cone 728′ reaches into therecess 728.

The base of the cone 728′ is fixedly received in a recess 717 formed ina slide-in plate 726′ of the lower constituent element 710′. Adisc-shaped damping element 715 made of an elastomer or anotherresilient material is sandwiched between a base of the cone 728′ and abase of the recess 717.

The needle joint makes it possible to swivel the lower constituentelement 710′ around a swivel point which is situated in the apex of therecess 728 formed in the upper constituent element 710. FIG. 26 showsthe intervertebral disc prosthesis of FIG. 25 in a state after swivelingthe lower constituent element 710′ by a few degrees. Since the recess728 is formed within the cap 716 of the upper constituent element 710,the swivel point is in immediate vicinity to the anatomical center ofmotion indicated by a dot 780.

The embodiment shown in FIGS. 25 and 26 differ from the embodimentspreviously described further in that the external surface 720 of the capinsert 716 is not spherical. Instead, this external surface 720 isspecifically adapted to the shape of the dome 723 formed between theapophyseal ring 771 of the upper vertebra LV1. To be more precise, theexternal surface 720 is formed as a complement part with respect to thedome 723, and there is no substantial gap between the external surface720 on the one hand and the dome 723 on the other hand. Preferably, thisgap has a width of less than 1 mm, at least over one half of the innersurface area of the dome 723.

Such an adaptation of the external surface 720 to the dome requires thatthe shape of the dome 723 is biometrically determined prior to theinsertion of the intervertebral disc prosthesis into the intervertebraldisc compartment. For determining the shape of the dome 723,high-resolution images of the upper vertebra L1 may be computerprocessed. The external surface 720 is then machined or molded inaccordance with the obtained biometrical shape data of the dome 723.

Adapting the shape of the external surface 720 to the adjacent dome 723has the advantage that the upper constituent element 710 is much morerigidly attached to the upper vertebra L1. Since the dome is not onlyaspherical, but usually also non-rotationally symmetrical, the capinsert 716, and thus the upper constituent element 710 as a whole,cannot rotate within the intervertebral disc compartment if there is asufficient pressure exerted on the intervertebral disc prosthesis by thesurrounding ligaments.

Further, there is no need to provide an additional rough coating thatensures a more intimate connection between the cap insert and the dome723, as is the case in the embodiments previously described. Even more,it may be advantageous to polish the external surface 720 to reduce itsroughness. This is because a smooth external surface 720 makes itpossible that the cap insert 716, and thus the entire upper constituentelement 710, adjusts itself with respect to the dome 723 by slidingmovements when the intervertebral disc prosthesis is inserted into theintervertebral disc compartment. By exerting a slight longitudinalpressure, the constituent element 710 will slightly rotate around alongitudinal axis of the vertebrae VL1, VL2 until they reach a positionin which a perfect match of the external surface 720 and the dome 723 isachieved.

Of course, the same considerations apply also to the lower cap insert716′ and its external surface 720′. It is further to be understood thatspecifically adapting the shape of the convexly projecting parts of theconstituent elements of the intervertebral disc prosthesis may beadvantageously used not only in connection with the present embodiment,but also with the other embodiments and quite generally with allprostheses having at least one convexly projecting part that reachesinto the dome of an adjacent vertebra.

FIGS. 27 and 28 show, in representations similar to FIGS. 25 and 26,still another embodiment of an intervertebral disc prosthesis in twodifferent swiveling states. In this embodiment the space between theupper and lower constituent elements 810, 810′ is filled with aring-shaped damping disc 819 having resilient properties. The disc 819is preferably stiff enough to carry some of the substantial forces thatare present between the adjacent vertebrae LV1, LV2. On the other hand,the disc 819 has to be resilient enough to enable swiveling movements ofthe lower constituent element 810′ with respect to the upper constituentelement 810.

The disc 819 has therefore the advantage that, on the one hand, itreduces the forces exerted on the cone 828′ and the recess 828, and onthe other hand it provides for an efficient limitation of the swivelingmovements. In comparison to the projections 738, 738′ serving as a stopin the previous embodiment, the disc 819 ensures a smooth limitation ofthe swiveling movement, because the resilient forces increase withgrowing swiveling angles.

It can be further discerned in FIGS. 26 and 27 that the anatomicalcenter of motion 880 is now positioned exactly in the apex of the recess828. This is achieved by abrasing a part of the upper lumbar vertebraLV1 to an extent that is indicated in FIGS. 27 and 28 by a dashed line821, which represents the shape of the upper lumbar vertebra LV1 priorto the abrasive process. The height of the removed bone volumecorresponds, at least substantially, to the thickness of the supportingplate 812 above the apex of the recess 828. This height may be in therange between 1 mm and 5 mm and is preferably between 1.5 mm and 2.5 mm.

FIGS. 29 and 30 show an upper constituent element 910 of anintervertebral disc prosthesis according to still another embodiment ina top view and a section along line XXX-XXX, respectively. The upperconstituent element 910 mainly differs from the other upper constituentelements described above in that it comprises a cap insert 916 having ashape that is specifically adapted to the recess within the apophysealring of the adjacent vertebra. More particularly, the cap insert 916 hasa shape that is at least substantially complementary to this recess.This means that the gap between the cap insert 916 and the bone tissuebetween the apophyseal ring is very small, preferably not exceeding 1 or2 mm. Preferably the largest part of the convex surface of the capinsert 916 is in direct contact to this bone tissue. However, this doesnot mean that the cap insert 916 has to bear the large forces exerted bythe adjacent vertebrae. Instead, the upper constituent element 910comprises a supporting plate 912 having a flat top annular area 923 onwhich the apophyseal ring of the adjacent vertebra rests. The supportingplate 912 is preferably made of a hard material having a rough surface.

In the embodiment shown, the upper constituent element 910 is assembledby inserting the cap insert 916 from below into the supporting plate 912until it abuts on an annular projection 913. The cap insert 916 is thensecured by means of a plate 915 which has a thread (not shown) on itscircumference so that it can be screwed into the supporting plate 912.On the underside of the plate 915 a slide-in plate 926 with a convexspherical-cap joint is inserted in a similar way as shown in FIGS. 1 and2.

FIGS. 31 and 32 show the cap insert 916 in an enlarged view and in asection along line XXXII-XXXII, respectively. The cap insert 916 has, ina cross section along its longitudinal axis, a substantially horizontalsection 931 centered between two inclined sections 933, 935. The capinsert 916 has, in a cross section along its transversal axis, a steeplyinclined first section 937 and a less steeply inclined second section939. This shape of a ramp corresponds to the shape of the dome formedbetween the apophyseal ring of the adjacent vertebra.

FIGS. 33 and 34 show a cap insert 1016 according to another embodimentin illustrations similar to FIGS. 31 and 32, respectively. Again theconvex region of the cap insert 1016 has the shape of a ramp similar tothe embodiment shown in FIGS. 31 and 32. However, in this embodiment theconvex region of the cap insert 1016 has a shape that slightly deviatesfrom the shape of the dome between the apophyseal ring of the adjacentvertebra. This will be explained in more detail with reference to FIGS.35 a to 35 c and 36 a to 36 c further below.

Returning back to FIGS. 33 and 34, the cap insert 1016 has a main flank1060 which is inclined with respect to a base area 1062 by an main flankangle δ₁ that may be, at least for most vertebrae, in a range between 5°and 20°. A counter flank 1064 is arranged opposite the main flank 1060and forms a counter flank angle δ₂ with the base area 1062. The counterflank angle δ₂ may be, at least for most vertebrae, in a range between25° and 50°. In the embodiment shown there is a transition area 1066between the main flank 1060 and the counter flank 1064 which does notcontain any edges and thus smoothly connects the main flank 1060 withthe counter flank 1064. As can be seen in FIG. 33, the ramp of the capinsert 1016 has lateral sides 1068, 1070 that smoothly slope downtowards the base area 1062.

The top surface of the cap insert 1016 is provided with a top coating1073. The top coating 1073 should provide low friction relative to thesoft bone material (substantia spongiosa) within the apophyseal ring ofthe adjacent vertebra. In an advantageous embodiment top coating 1073 ismade of a diamond-like carbon (DLC). Such a coating can be applied usingion beam deposition or sputter deposition techniques to the surface ofthe cap insert 1016 and is not only biologically compatible, but is veryhard and provides a smooth surface with very low friction. An arithmeticroughness Ra of less than 10 μm or even less than 1 μm may be easilyachieved with such a coating 1073, which results in a very low frictionto the adjacent bone material.

Instead of applying a low friction coating to the cap insert 1016, itssurface may be polished by conventional methods. A polished surface oftitanium, for example, also ensures very low friction values.

FIG. 35 a shows the cap insert 1016 in a section similar to FIG. 33 andthe lower dome D1 of the adjacent upper vertebra V1 in the same section.As can be easily seen from FIG. 35 a, the shape of the dome D1 slightlydiffers from the shape of the cap insert 1016.

If the cap insert 1016 is inserted into the dome D1 of the upperconstituent element 910 in a longitudinal direction indicated by anarrow 1074, only a small area in the vicinity of the vertex of the capinsert 1016 gets into contact with the vertex of the dome D1. Thisconstellation is shown in FIG. 35 b. As a result of the low frictioncoating 1073 applied to the surface of the cap insert 1016, the lattermay easily be rotated by at least 10°, preferably by at least 25°,within the dome D1 until it has reached its final rotational position.The ability to rotate is denoted in FIG. 35 b by a double arrow 1076.Easy rotatability is particularly important if the prosthesis isinserted through a narrow access canal with its longer dimension alignedalong the canal axis. This even holds true for ventrolateral accesscanals because even there the constituent elements of the prosthesishave to be rotated by some 20° before they reach their final implantpositioned.

The rotation may be achieved by means of the rods 98, 100 shown in FIGS.23 a to 23 c or by any other suitable means that are separablyconnectable to the upper constituent element 910. In certain cases noactive manipulation at all, or a least no manual fine adjustment, isnecessary because a self-centering effect occurs due to the low frictioncoating 1073 and the rotationally asymmetric shape of the dome D1 andthe cap insert 1016. The term “self-centering effect” means that the capinsert 1016, and thus the entire upper constituent element 910, rotatesif a compressive force is applied between the supporting plate 910 andthe adjacent vertebra. During the implant surgery this force is producedby ligaments that extend along the spine. When the supporting plate hasrotated to a position where these compressive forces are symmetrical,the torque causing the rotation vanishes, and the rotation ceasesaccordingly.

The self centering effect becomes very prominent if the kinetic frictioncoefficient observed between the coating 1073 and the dome D1 is as lowas 0.1

If the movement of the upper constituent element 910 along direction1074 continues due to tension forces applied by ligaments to theadjacent vertebrae V1, V2, the vertex area of the cap insert 1016 thatcome first into contact with the dome D1 deforms the dome D1 bydisplacing some of the soft bone tissue (substantia spongiosa). Thisprocess continues until the hard apophyseal ring 1079 of the adjacentvertebra V1 rest on the annular area 923 of the supporting plate 912.This final constellation, in which a larger area or even the most partof the cap insert 1016 has intimate contact with the soft bone tissue(substantia spongiosa) of the upper vertebra V1, is shown in FIG. 35 c.A thin dashed line 1078 indicates the shape of the dome D1 prior to itsdeformation by the cap insert 1016.

As a result of the rough surface of the annular area 923 and theintimate contact between a large part of the cap insert 1016 with theadjacent dome D1 the upper constituent element is rigidly fixed to theupper vertebra V1 so that no further fixing means like screws arerequired for maintaining the upper constituent element 910 in its finalposition.

FIGS. 36 a to 36 c show illustrations similar to FIGS. 35 a to 35 c, butfor the cross-section as shown in FIG. 34.

Although the cap insert 1016 has a shape that slightly deviates from theshape of the dome D1, it may be advantageous to manufacture the capinsert 1016 individually for each vertebra on the basis of biometricdata obtained for the particular patient. This ensures optimalrotatability on the one hand and a good intimate contact between the capinsert 1016 and the dome D1 on the other hand. However, sincecorresponding vertebrae of the human vertebral column often have similarshapes, it may suffice to provide the surgeon with a set of differentcap inserts from which he selects one that suites best to the shape ofthe dome that has been biometrically determined beforehand.

In the embodiments described above it has been assumed that the capinsert is a separate part having different material and/or surfaceproperties. However, the provision of a separate cap insert may also beadvantageous with cap insert that are individually adapted to thepatient's biometric data shall be manufactured. In this case only theindividually manufactured cap inserts can be inserted into one or a fewdifferent supporting plates having the same standard recess forreceiving the cap insert.

As a matter of course, it is also possible to manufacture supportingplates with integrated cap as a single piece. FIG. 37 shows, in arepresentation similar to FIG. 30, an upper constituent element 1110having supporting plate 1112 with an integrated cap 1116. The surface1173 of the cap 1116 is polished so that an arithmetic roughness Ra ofless than 10 μm 0.1 is achieved, resulting in a kinetic frictioncoefficient of less than 0.1 with respect to soft bone material(substantia spongiosa).

In this embodiment a slide-in plate 1126 with a convex spherical-capjoint is received in a plate 1115 having elastic material properties.The material may have a modulus of elasticity of more than 1500 N/mm²which results in very good shock absorbing properties. Apart from theelastic plate 1115 also allows for translational movements of theslide-in plate 1126. As a result, the constituent elements of theprosthesis may, at least to a small extent, be translationally shiftedrelative to one another. The superposition of translational movementsand of swivel movements provided by the ball-and-socket joint optimallyreproduce the movements of healthy vertebrae of the vertebral column. Itis to be understood that the provision of a plate 1115 having elasticmaterial properties may also be advantageous in any of the otherembodiments described above.

The above description of the preferred embodiments has been given by wayof example. From the disclosure given, those skilled in the art will notonly understand the present invention and its attendant advantages, butwill also find apparent various changes and modifications to thestructures and methods disclosed. The applicant seeks, therefore, tocover all such changes and modifications as fall within the spirit andscope of the invention, as defined by the appended claims, andequivalents thereof.

The invention claimed is:
 1. An intervertebral disc prosthesis forinsertion into an intervertebral disc compartment which is formedbetween a first vertebra and a second vertebra, the prosthesiscomprising: a) a first constituent element which comprises a firstsupporting plate, in which, on one side, a recess is formed and whichhas an annular flat region surrounding the recess and abutting againstthe first vertebra when the intervertebral disc prosthesis is insertedinto the intervertebral disc compartment, comprises an insert which isreceived in the recess and which has a convexly curved region having atleast substantially the shape of a ramp, and comprises a first jointmember arranged on an opposite side of the first supporting plate, andb) a second constituent element which comprises a second supportingplate, in which, on one side, an abutment face is formed which abutsagainst the second vertebra when the intervertebral disc prosthesis isinserted into the intervertebral disc compartment, and comprises asecond joint member arranged on an opposite side of the secondsupporting plate, wherein the convexly curved region has a longdimension in a first direction and a short dimension in a seconddirection orthogonal to the first direction, wherein the convexly curvedregion comprises, in a cross section perpendicular to the firstdirection, a first inclined section having a first incline and a secondinclined section having a second incline smaller than the first incline,and wherein the convexly curved region has, in a cross sectionperpendicular to the second direction, a substantially horizontalsection centered between two inclined third sections.
 2. Theintervertebral disc prosthesis according to claim 1, wherein the recessis formed by an opening provided in the first supporting plate.